Here's the model, note that the FileExtensions attribute decorates the sole property on the model:

    using System.ComponentModel.DataAnnotations;

    namespace jQuery.Validation.Unobtrusive.Native.Demos.Models
    {
        public class FileExtensionsModel
        {
            [FileExtensions(Extensions = "txt,pdf")]
            public string FileExtensions { get; set; }
        }
    }
        

Here's the view (which uses the model):

    @model jQuery.Validation.Unobtrusive.Native.Demos.Models.FileExtensionsModel
    @using (Html.BeginForm())
    {
        <div class="row">
            @Html.LabelFor(x => x.FileExtensions, "Only a text file or PDF can be selected:")
            @Html.TextBoxFor(x => x.FileExtensions, true, htmlAttributes: new {type = "file"})
        </div>

        <div class="row">
            <button type="submit" class="btn btn-default">Submit</button>
            <button type="reset" class="btn btn-info">Reset</button>
        </div>
    }
        

Here's the HTML that the view generates:

    <form action="/Demo/Email" method="post">
        <div class="row">
            <label for="FileExtensions">Only a text file can be selected:</label>
            <input data-msg-extension="The FileExtensions field only accepts files with the following extensions: .txt, .pdf" data-rule-extension="txt,pdf" 
                id="FileExtensions" name="FileExtensions" type="file" value />
        </div>
        <div class="row">
            <button type="submit" class="btn btn-default">Submit</button>
            <button type="reset" class="btn btn-info">Reset</button>
        </div>
    </form>
        

Here's the JavaScript that initialises the validation:

  <script type="text/javascript" src="/Scripts/additional-methods.min.js"></script>

  $("form").validate({
      submitHandler: function (form) {
          alert("This is a valid form!");
      }
  });